Signal clamp for a dual function input terminal of a one-chip television signal processing IC

摘要

A so-called "one-chip" television IC includes a dual purpose input terminal for receiving a control signal representing an image property, such as a contrast, and also a signal having an amplitude within a predetermined range for enabling a test mode of operation. The control signal is generated by at least first and second control circuits. The first control circuit provides a first control signal set by a user. The second control circuit provides a second control signal in response to the image content of an image-representative signal produced by the integrated circuit, and may comprise a beam current limiter responsive to the beam current drawn by an image reproducing device or a circuit responsive directly to the image-representative signal. The first and second control signals are combined to produce a combined control signal which is applied to the dual purpose input terminal. A circuit is coupled to the dual purpose input terminal to inhibit a voltage which extends into the predetermined range corresponding to the test mode from being developed at the terminal in response to the combined control signal.